Imagens das páginas
PDF
ePub

thing to do with the nutrition of plants; and that, in those cases where it was met with, it was merely in a state of separation from the atmospheric air which had been inhaled and deprived of oxygen and carbonic acid. But its constant presence in combination with the tissue of Mushrooms and of Cruciferous plants, in gluten, and what chemists call vegetable albumen, and also in vegetable alkalies, seems a sufficiently strong proof of its contributing, in some way or other, to the nutrition of the vegetable system.” And M. Boussingault has shown that it is in fact a constant element of vegetation, most concentrated in seeds, to the maturation of which it is essential, and dispersed through the other parts of the tissue. (Comptes rendus, vi. 105.)

Fixed as plants are to the soil, deprived of volition, and incapable of removing their highly absorbent roots from what is hurtful to them, except with extreme slowness, it appears scarcely probable that they should have any power of selecting their food; on the contrary, the facility with which they are poisoned would seem to confirm the correctness of the usual supposition. But, if roots are made to grow in coloured infusions, it is said that they take up only the colourless parts, leaving the coloured behind; and we know that if an apple tree is planted in a piece of ground in which another apple tree has been growing many years, the new plant will languish and become unhealthy, whatever quantity of manure, that is of new food, may be offered to its roots. This last fact is accounted for upon the supposition that the soil contains some peculiar principles which are necessary to the health of an apple tree, and that the old tree, having selected for its own consumption all that the soil contained, has left none behind it for the new comer; but the probability is, that this hypothesis is untenable, and that the fact is to be explained upon very different principles (see Chap. X.). It has been, however, demonstrated by Daubeny, that plants have, to a certain extent, a power of selection by their roots. He found that when barley was watered with distilled water, containing in every two gallons two ounces of nitrate of strontian, not a trace of that earth could be detected in the ashes of the plants; and when Lotus tetragonolobus was treated in a

BB

similar manner, except that only two ounces of nitrate of strontian were dissolved in ten gallons of distilled water, although the whole of that quantity was expended upon them, a minute examination demonstrated that the stems contained no trace whatever of strontian, although a small portion appeared to be present in, or at least adherent to, the roots. By other experiments it was ascertained, that the strontian was not in these cases first received into the system, and afterwards rejected through the roots; for when the roots of a Pelargonium were divided into two nearly equal bundles, one of which had its extremity immersed in a glass containing a weak solution of nitrate of strontian, the other in one containing pure distilled water, after the lapse of a week the water in the second glass was tested, but no strontian could be discovered in it, although a single grain in one pint would have been readily detected. Hence it appears, “ that plants do possess, to a certain extent at least, a power of selection by their roots, and that the earthy constituents which form the basis of their solid parts are determined as to quality by some primary law of nature, although their amount may depend upon the more or less abundant supply of the principles presented to them from without.” (Linn. Trans. xvii. 266.)

It must be obvious, that the exhaustion of soil by plants means their having consumed all the nutritive particles that it contains. Whether this means all particles that are capable of forming carbonic acid is, however, not so certain : it is highly probable that other matters are equally indispensable to the health of particular plants; as, for example, of Corn. Corn cannot remain in health, unless it has the power of attracting fluid silex from the earth, and of consolidating it in its epidermis. It is to be supposed, that the presence of alkaline principles in the soil is necessary to render the siliceous matter soluble; therefore, to exhaust a soil of alkaline principles would be to render it unfit for the support of Corn; and, consequently, alkaline principles may be considered nutritive in regard to Corn: and so of other things.

Again, Thaer and Boussingault both agree in considering the efficiency of manures dependent in a great measure upon their animalised nature, or their power of adding nitrogen to vege

table tissue; and, consequently, it is probable that the exhaustion of soil does not depend merely upon the destruction of carbonaceous matter, but also upon the consumption of the azotised matter contained in it. This is a most important fact to consider, in attempting to estimate the action of manures. (Comptes rendus, vi. 106.) M. Payen asserts that every nascent, or developing, organ contains nitrogen in abundance, and that, as a given organ developes, the azotised matter diminishes in proportion to the unazotised, which by degrees becomes predominant. (Ibid. vi. 132.) It is, therefore, essential for plants to be placed in such circumstances as may give them the power of assimilating nitrogen.

This diminishes the complicated nature of the theory of manures, and the seeming impossibility of reducing it to any fixed and intelligible laws. But, ignorant as we are of most of the more obscure phenomena that are attendant upon vegetable life, unacquainted with the action of a large proportion of the principles that the chemist discovers among the tissue of plants, and incapacitated by our limited means of observation from watching any except the most obvious and general properties of living vegetable matter, we cannot expect, in such a state of things, to arrive at any precise ideas as to what kind of food or stimulants exercises the most energetic and wholesome influence upon plants. I accordingly feel no surprise at the statement of a friend of mine, well known alike for his agricultural skill, his chemical knowledge, and his remarkable good sense, “ that chemistry has hardly advanced the art of agriculture a single step, but that the latter remains, after all the investigations of the chemists, a mere empirical art.”

Those who wish to understand the modern opinions concerning the action of manures (properly so called) should consult De Candolle's Physiologie, p. 1278., and the papers of Payen, Boussingault, Thaer, &c.

CHAPTER XII.

OF DIGESTION, RESPIRATION, AND SECRETION.

AFTER the food is received into the system of a plant, it is gradually conveyed into the leaves, where it becomes decomposed or digested. It is probable that, in its passage through the stem, it undergoes some kind of decomposition, leaving a portion of its water and carbon fixed among the tissue; but it is principally in the leaves that it is altered. By the time, however, that it has arrived in these organs, it is by no means in the same state as when it entered the roots; but it becomes altered in its nature, and in its specific gravity, by the addition of what soluble matter it meets with in its progress, as has been proved experimentally by Knight.

The alteration that the fluids of plants undergo in their leaves appears to consist in parting with superfluous water by evaporation; in decomposing water and carbonic acid; and in assimilating the various matters which are left behind. The causes of these actions are believed to be, light, and the atmospheric dryness which light produces.

According to De Candolle, it is light alone to which evaporation and the suction of fluids by the roots are to be assigned. He says: “ If you select three plants in leaf, of the same species, of the same size, and of the same strength, and place them in close vessels, one in total darkness, the other in the diffused light of day, and the third in the sunshine, it will be found that the first pumps up very little water, the second much more, and the third a great deal more than either. These results vary according to species and circumstances; but it uniformly happens that plants in the sun absorb more than those in diffused light, and the latter more than those in darkness; the last, however, pumping up something. If, again, we take three similar plants, and, preventing their absorption by the roots, after weighing them carefully, place them in three similar situations, we shall find that that exposed to the sun has lost a great quantity of water, that in common daylight a less amount, and that which was in total darkness almost nothing."

It is, however, to be supposed, that light is, to a certain extent, in these cases, a remote, as well as immediate, cause of evaporation: for we cannot apply solar light to plants without heating and rarefying their atmosphere. It is a well known fact, that plants perspire in a sitting-room the air of which is constantly dry, but which is but imperfectly illuminated, so much more than in the open air exposed to the direct rays of the sun, that it is impossible to keep many kinds of plants alive in such a situation.

Light is, however, to all appearance, the exclusive cause of the decomposition of carbonic acid. It was long since remarked by Priestley, that, if leaves are immersed in water and placed in the sun, they part with oxygen. This fact has been subsequently demonstrated by a great number of curious experiments, to be found in the works of Ingenhouz, Saussure, Senebier, and others. Saussure found that plants in cloudy weather, or at night, inhaled the oxygen of the surrounding atmosphere, but exhaled carbonic acid if they continued to remain in obscurity. But, as soon as they were exposed to the rays of the sun, they respired the oxygen they had previously inhaled, in about the same quantity as they received it, and with great rapidity. Dr. Gilly found that grass leaves exposed to the sun in a jar for four hours produced the following effect:At the beginning of the experiment | At the close of the experiment there there were in the jar :

were :Of nitrogen - 10:507 | Of nitrogen

- 10:507 Of carbonic acid - - 5•7 Of carbonic acid - - •37 Of oxygen . . . 2.793 Of oxygen - - - 7.79

[blocks in formation]

Heyne tells us that the leaves of Bryophyllum calycinum, in India, are acid in the morning, tasteless at noon, and bitter in the evening; Link himself found that they readily stained litmus paper red in the morning, but scarcely produced any

« AnteriorContinuar »